1. Field of the Invention
This invention relates to a stepping motor, and more particularly to a stepping motor of the permanent magnet type.
2. Description of the Prior Art
Various types of stepping motors are conventionally known, and an exemplary one of such conventional stepping motors is constituted such that a pair of stator coils which form first and second phases are disposed along a direction parallel to a rotary shaft of the stepping motor.
A stepping motor of the type just mentioned is shown in FIG. 8. The stepping motor shown is a two-phase permanent magnet type stepping motor and includes a stator 2B which forms a first phase (hereinafter referred to as A phase), an A phase coil 2A wound on the stator 2B, a B phase stator 3B which forms a second phase (hereinafter referred to as B phase) and is disposed adjacent the A phase stator 2B in a direction parallel to a rotary shaft 13, and a B phase coil 3A wound on the B phase stator 3B.
A permanent magnet 12 is secured to the rotary shaft 13 and has a size sufficient to oppose the entire A phase coil 2A and B phase coil 3A. The permanent magnet 12 and rotary shaft 13 are supported for rotation by means of a pair of bearings 8 and 9 and a pair of housings 5 and 6.
A permanent magnet type stepping motor having such construction as described just above, however, has a drawback since a torque attracting the rotor to the stators (such torque will be hereinafter referred to as detent torque) is produced. The detent torque is produced when the coils are not energized with exciting currents and is caused by magnetic fluxes generated from the permanent magnet. The torque (holding torque) of the stepping motor when the coils are energized is distorted in waveform, which causes an increase in production of vibrations and acoustic noises.
Further, since the detent torque itself then acts as a load to the stepping motor, the output torque of the stepping motor is decreased as much. When the stepping motor is of a small size, such a construction is employed so that a strong permanent magnet of a rare earth element is adopted or the air gap between the stator and the rotor magnet is minimized. Consequently, there is a problem that the detent torque is increased as much so that the stepping motor cannot rotate smoothly.
Various solutions to the problem have been proposed. According to an exemplary one of such solutions, a 2-phase stepping motor is constructed such that a pair of A and B phase coils are disposed in a juxtaposed relationship along a direction of a rotary shaft and are displaced by 90 degrees by electric angle from each other. A cylindrical permanent magnet is secured integrally to a rotary shaft of the motor and is formed such that opposing faces thereof to an A phase stator and a B phase stator have an equal area so that the detent torque which is a second harmonic wave is cancelled by a combination of the A and B phases.
However, the magnitudes and shapes of waveforms of torque by A and B phases are different among individual stepping motors due to an influence of a saturated condition of a stator yoke, irregularity in magnetization of a magnet, accuracy in assembly of a stator and so forth. Consequently, it is difficult to cancel a second harmonic wave even if torques TA1 and TA2 by the A phase and torques TB1 and TB2 by the B phases are combined as seen from FIG. 7. Thus, the stepping motor has a problem in that a detent torque DT appears significant as it remains as a second harmonic wave, and the solution is not sufficient.